(1) Field of the Invention
The present invention relates generally to video data transmission systems, and more particularly to the systems and methods of measuring audio/video synchronization television systems.
(2) Description of the Prior Art
The loss or degradation of audio to video synchronization, or A/V sync, in television systems is a well-known problem. Loss of A/V sync can be attributed to the separate processing and hence delays of the video and audio components of the broadcast. The advancement of television technology has resulted in increased processing of the video to the point where the degree of audio/video skew is highly objectionable to the viewer. In recent years, the application of MPEG-2 compression on the video and audio signals has further exacerbated the problem.
It is standard operating procedure in the broadcast industry to measure A/V sync and apply delays, usually in the audio path, to resynchronize the program audio and video. The A/V sync test involves simultaneously sending an audio and a video test signal and measuring the difference in arrival time. Although the video signal may be concealed from viewers by inserting it within the vertical blanking interval (VBI), the audio test signal cannot be concealed and it is transmitted as part of the audio program material. Because this highly noticeable audio tone interferes with the program material, broadcast engineers are very reluctant to perform A/V sync testing in-service. When faced with an out of A/V sync condition during a live broadcast, engineers will attempt to remedy the situation by adjusting the audio synchronizer while judging audio/video sync by eye. Only as a last resort is an A/V test run in-service.
The introduction of MPEG-2 compression to transmit video over a digital network has worsened the A/V sync problem in two respects. Firstly, the amount of processing performed on the video portion of the broadcast during the encoding process is enormously greater than the audio. Secondly, MPEG-2 encoders do not transmit test signals located in the VBI due to bandwidth considerations. This requires a test engineer to transmit the video sync marker signal within the active viewing area where it may be detected by viewers. So by deploying MPEG-2 compression technology, the engineer is faced with the quandary of an increased need for A/V sync tests and being unable to conceal even the video portion of the test signal.
IBM Video Systems (IVS) operates a high-bandwidth, switched network connecting 6 cities in the continental United States that is used by broadcasters to transmit and receive broadcast video. This service converts a subscriber""s analog video to compressed digital format, then routes it to the destination over an asynchronous transfer mode (ATM) switched connection where the digital video is decompressed, converted back into analog and passed on to the receiving end. The analog video is digitized and compressed using MPEG-2 encoding format at a bit rate of 8-40 Mbps. The video is encoded and decoded in real-time and, in many cases, played out directly to air.
The MPEG-2 compression and ATM network, unique to IVS, can cause perturbations that are not normally seen in an analog network. To detect network anomalies and failures real-time, in-service circuit testing is performed so that service may be restored with minimum circuit outage.
Broadcast engineers are a demanding customer set with exacting standards for video quality and availability of service. Since the subscriber""s feed is broadcast directly to air via the IVS network, any degradation or interruption of video signal will be obvious to television viewers and may result in a significant loss of revenue to the broadcaster.
Prior art related to A/V synchronization includes: U.S. Pat. No. 5,243,424 issued Sep. 7, 1993, filed May 16, 1991 (Emmit) teaches a method of non-intrusive A/V sync testing by using a tonal audio marking signal that is sufficiently reduced in amplitude to render the tone inaudible however still detectable electronically in order to perform the measurement. The apparatus relies on the existing program material to mask the tone. Instead of injecting a video marker, a technique of waiting for a cut in the video program is taught which causes the audio marker to be injected exactly one second later. The use of a tone as an audio marker mimics the traditional method of A/V sync testing and represents no advantage over prior art. The technique of reducing the audio marker signal strength to an inaudible level has not gained acceptance in the industry because it results in either the audio marker being undetectable or being falsely detected. Furthermore, the psycho-acoustical filter techniques of MPEG-2 audio encoding are specifically designed to mask (i.e., not encode) inaudible side tones present in the audio program which renders this method unsuitable for transmission systems using MPEG-2 compression.
U.S. Pat. No. 5,555,364 issued Jan. 21, 1997, filed Mar. 30, 1995 (Wolf) discloses a device for perception-based, in-service measuring of A/V sync across a transmission system. The device located at the transmitting end extracts features from the video and audio program and time stamps them. The video features are created by detecting inter-frame motion. The device at the destination computes and time stamps the same video and audio features. A separate communication link is used to transfer the origin features and timestamps to the destination site where they are processed along with the destination features and timestamps to determine the end-to-end video and audio delay. The two delays are then subtracted to calculate A/V sync. A disadvantage of the system is that A/V sync cannot be calculated unless motion is present in the video program. Furthermore, the A/V sync measurement does not provide a precise, absolute measurement, but rather a measurement that xe2x80x9cis consistent with human perception.xe2x80x9d
U.S. Pat. No. 5,040,081 issued Aug. 13, 1991, filed Feb. 16, 1989 (McCutchen) discloses a video or film real time synchronization to audio of sequences of images without the need to have a separate reference or carrier. Master/slave control is accomplished via a variable speed SMPTE time code generator.
U.S. Pat. No. RE 33,535 issued Feb. 12, 1991, filed Oct. 23, 1989 (Cooper) teaches a method of synchronizing audio and video by encoding the audio onto the video prior to transmission. On the receive end, the embedded audio is removed from the video signal and the proper delay is inserted.
U.S. Pat. No. 4,963,967 issued Oct. 16, 1990, filed Mar. 10, 1989 (Orland) discloses an out-of-service A/V sync test using a video pulse and an audio tone that is displayed on an oscilloscope. A synchronizer delays the audio unit markers until they coincide.
U.S. Pat. No. 4,313,135 issued Jan. 26, 1982, filed Jul. 28, 1980 (Cooper) teaches a method of preserving A/V sync by using an audio synchronizer to detect delay and adjust it using a variable frequency clock. The video color framing field is used to time video.
U.S. Pat. No. 5,572,261 issued Nov. 5, 1996, filed Jun. 7, 1995 (Cooper) teaches A/V sync measurement by inspection of the opening an closing of the mouth of the speaker and comparing that opening and closing to the utterance of sounds associated therewith.
None of the prior art discloses or suggests a non-intrusive, non-service affecting audio/video synchronization test for which a clear need exists in the industry.
An object of the invention is an in-service apparatus and method of non-intrusively measuring audio/video synchronization of a television broadcast across an analog, digital, or compressed digital transmission channel.
Another object is to measure A/V sync without using traditional audio marker tones.
These objects and other objects, features and advantages are accomplished in a network providing high bandwidth, broadcast quality video and audio signals. In one embodiment, the analog and video signals are digitized and compressed. The compressed digitized A/V signal is transmitted via an ATM network to a distant end where the signal is converted back into analog or digital format and passed on to the subscriber. The network includes an A/V synchronous test signal generator which injects video and audio markers into the video and audio non-intrusively and routes the two signals into a switch where they are switched into a channel for encoding and transmission via the ATM network. At the distant end the signal is decoded and routed by a switch into the A/V test generator set where the markers are detected and the A/V skew calculated, after which the audio and video are routed to the subscriber. The A/V test set signal generator includes a Video Blanking Interval (VBI) test signal generator and a white noise generator, the former injecting. a marker into the video signal and the later injecting an audio marker into the audio signal. The video marker is injected into the VBI and broadband, background audio noise to measure the delay between the audio and video components of a broadcast. The video marker is a single line of white injected into VBI for a duration of video one frame or 33 milliseconds. The marking of the audio is accomplished by gradually injecting white noise into the audio channel until the noise level is 6 dB above the noise floor of the audio receiver. The white noise is effectively masked by the audio program. When a period of quiet occurs in the audio program, the injected white noise is removed for a period of 1 frame which is detected by the measurement test set as the audio marker. The injected white noise is then slowly attenuated, then removed to escape notice by the viewer/listener. So that the measurement test set at the distant end may unambiguously recognize the injected white noise as the precursory A/V sync signal, a small spectrum of the white noise is notched or removed. This signature and its removal for precisely 1 frame time precludes inadvertent recognition of program audio noise as the audio marker. The noise floor of a broadcast quality audio decoder or the receiver is typically very low, in the range of xe2x88x9258 to xe2x88x9270 dBu, which is inaudible on a consumer television set at normal listening levels. Even if the addition of 6 dB of white noise were to cross the threshold of audibility, it is not perceived by the listener because it is not abruptly added or removed and it cannot be heard over the audio program. Removal of the white noise for a period of 1 frame is psycho-acoustically inaudible to the human ear in normal listening environments. Periods of black video and audio quiet are inserted between program segments and interstitial (e.g., commercials, promotions, public service announcements). This audio quiet or xe2x80x9caudio blackxe2x80x9d is actually the absence of audio program which causes the television audio level to fall to the noise floor level of the audio decoder or receiver in the transmission channel. It is during these brief periods of audio black that A/V synchronization may be imperceptibly tested.
As a further improvement, an A/V sync may also be tested in situations where interstitial are infrequent by testing during periods of quiet during the audio program. Each audio program has its own noise floor that is somewhat higher than the audio decoder noise floor. The audio program noise floor is often established by the ambient noise picked up by the microphones in a broadcast studio or stage setting when no one is talking and there is no other background noise such soft music. To facilitate the testing of A/V sync during periods audio program quiet, the A/V test signal generator monitors the audio program to establish a noise floor and measures A/V sync by injecting white noise 6 dB above this noise floor. A maximum noise floor may be specified to prevent the injected noise from rising to a clearly obvious and objectionable level.